RFC 988 (rfc988) - Page 2 of 20
Host extensions for IP multicasting
Alternative Format: Original Text Document
RFC 988 July 1986 Host Extensions for IP Multicasting Multicast agents are also responsible for internetwork delivery of multicast IP datagrams. When sending a multicast IP datagram, a host transmits it to a local network multicast address which identifies all neighboring members of the destination host group. If the group has members on other networks, a multicast agent becomes an additional recipient of the local multicast and relays the datagram to agents on each of those other networks, via the internet gateway system. Finally, the agents on the other networks each transmit the datagram as a local multicast to their own neighboring members of the destination group. This memo specifies the extensions required of a host IP implementation to support IP multicasting, where a "host" is any internet host or gateway other than those serving as multicast agents. The algorithms and protocols used within and between multicast agents are transparent to non-agent hosts and will be specified in a separate document. This memo also does not specify how local network multicasting is accomplished for all types of network, although it does specify the required service interface to an arbitrary local network and gives an Ethernet specification as an example. Specifications for other types of network will be the subject of future memos. 3. LEVELS OF CONFORMANCE There are three levels of conformance to this specification: Level 0: no support for IP multicasting. There is, at this time, no requirement that all IP implementations support IP multicasting. Level 0 hosts will, in general, be unaffected by multicast activity. The only exception arises on some types of local network, where the presence of level 1 or 2 hosts may cause misdelivery of multicast IP datagrams to level 0 hosts. Such datagrams can easily be identified by the presence of a class D IP address in their destination address field; they should be quietly discarded by hosts that do not support IP multicasting. Class D addresses are defined in section 4 of this memo. Deering
